Dual-speed direct current (DC) motors are used in a number of applications. Uses of dual-speed DC motors may range from use in computer disk drives to sewing machines. DC motors are often used in portable devices such as laptop computers in an effort to maximize battery efficiency. In the auto industry, DC motors are typically used to operate windshield wipers and radiator fans. Effective and precise control of the dual-speed motor aids in increasing battery efficiency and motor life.
FIG. 1 illustrates a mechanical relay system (100) used in traditional dual-speed DC motor (110) control. As shown in FIG. 1, two mechanical relays (140, 130) were traditionally used. The first relay (130) is used to control the ON/OFF of the motor (110) by selectively providing DC power from the DC power supply (120). The second relay (140) illustrated in FIG. 1 is used to control the Hi/Low-speed of the motor (110). While the traditional mechanical relay system (100) used in FIG. 1 enabled dual-speed DC motor control, the size of the relays (130, 140) used in the traditional mechanical relay system (100) is bulky and the reliability and lifespan of the mechanical relays is poor.
With the development of semiconductor technology, mechanical relays are being replaced by solid-state switches. Solid-state switches are replacing mechanical relays due to their low cost, long life span, and increased reliability when compared to the traditional mechanical relays. However, the use of solid-state switches does have a number of drawbacks as well. One drawback of using solid-state switches to replace mechanical relays is that the solid-state switches, either MOSFET type or IGBT type, can only block or switch off the current in a single direction. In the opposite direction, the body diode of the transistor will be always conducting even when the switch is operating in the OFF mode. Consequently, it is impossible to switch the solid-state switch OFF in the opposite direction. Due to the inability to block or switch a current OFF in both directions, the use of solid-state switches in dual-speed motor control has been limited.